1. Field of the Invention
The present invention relates to a plasma doping method, a plasma doping apparatus used therefor, and a silicon substrate formed using the same. In particular, the invention relates to a method of performing plasma doping that dopes an impurity into a surface of a solid sample, such as a semiconductor substrate or the like.
2. Description of the Related Art
As a technology for doping an impurity into a surface of a solid sample, there is known a plasma doping (PD) method that ionizes the impurity and dopes the ionized impurity into a solid at low energy (for example, see Patent Document 1).
Meanwhile, among methods of doping an impurity, an ion implantation method is most widely used at present. As will be apparent from Non-Patent Document 1, the plasma doping method is also described in ITRS2003 as a next-generation technology of ion implantation. The plasma doping method is different from the ion implantation method.
A technical difference between ion implantation and plasma doping will now be described in detail.
In the ion implantation method, an apparatus having the following configuration is used. The apparatus includes an ion source that generates plasma from gas, an analysis magnet that performs mass separation in order to select desired ions among ions extracted from the ion source, an electrode that accelerates the desired ions, and a process chamber that implants the accelerated desired ions into a silicon substrate. In the ion implantation, in order to implant the impurity shallow, it is preferable to set energy extracting ions from the ion source and acceleration energy small.
However, when the extraction energy is set small, the number of ions to be extracted is decreased. In addition, when the acceleration energy is set small, while an ion beam is transported from the ion source to a wafer, a beam diameter is widened due to a repulsive force by charges between the ions. Accordingly, the ion beam may collide against the inner wall of a beam line, and thus a large number of ions may be lost. For this reason, throughput of an implantation processing may be lowered. For example, when B+ ions are implanted, if the acceleration energy becomes 2 keV or less, the throughput starts to be lowered. Then, if the acceleration energy becomes 0.5 keV or less, the beam transportation itself may be difficult. Further, even though the acceleration energy is lowered to 0.5 keV, the B ions may be implanted at a depth of approximately 20 nm. That is, in case of forming an extension electrode having a thinner thickness, productivity may be lowered drastically.
In contrast, in the plasma doping method, an apparatus having the following configuration is used. The apparatus includes a plasma generation source that induces plasma into a cylindrical vacuum chamber, in which a silicon substrate can be disposed, a bias electrode, on which the silicon substrate is disposed, and a bias power supply that adjusts a potential of the bias electrode. That is, the apparatus has the configuration, in which the analysis magnet and the acceleration electrode are not provided, different from the apparatus used in the ion implantation. The bias electrode that serves as a plasma source and a wafer holder is provided in the vacuum chamber. Then, the ions are accelerated and introduced by a potential to be generated between the plasma and the wafer. With this configuration, since low-energy plasma can be directly used, a large amount of low-energy ions can be irradiated onto the wafer, compared with the ion implantation. That is, a dose rate is considerably high. For this reason, in the low-energy B-ion implantation, high throughput can be kept.
With the application of the plasma doping method, the inventors have developed a process technology that forms a source-to-drain extension electrode having a very small thickness and low resistance. This new process technology is known as a process technology that has particular effects (Non-Patent Document 2).
In this method, doping material gas, such as B2H6, that is introduced from a gas introduction port is plasmized by a plasma generation unit having a microwave waveguide and an electric magnet. Then, boron ions in plasma are supplied to a surface of a sample by a high-frequency power supply.
With the reduction in size and high integration of a semiconductor device, characteristics in an impurity doped region are very important. Among these, a dose (impurity doping amount) determines low resistance that is one of important elements in determining element characteristics. Accordingly, the control of the dose is very important.
If the plasma doping method is used, it can be seen that the source-to-drain extension electrode having a very small thickness and low resistance can be formed. However, a control method of the dose for controlling the element characteristics has not been developed yet. Up to now, a method that changes the dose by changing a plasma doping time has been tested, but this method does not obtain sufficient control precision and thus is unpractical.
In this situation, as a method that can increase safety by diluting toxic B2H6 having a serious risk to the human body as large as possible, can stably generate and keep plasma without degrading doping efficiency, and can easily perform the control of the dopant dose, the inventors has suggested the following method. In this method, B2H6 gas as a material containing an impurity to be doped is diluted with He gas having small ionization energy, then He plasma is generated earlier, and subsequently B2H6 is discharged (Patent Document 2). In this method, there has been suggested that the concentration of B2H6 gas is preferably less than 0.05%.
When the concentration is low, for example, approximately 0.05%, although it is not reported that the dose is easily controlled, the dose is changed by changing the time while the gas concentration is kept constant. That is, when the B2H6 gas concentration is low, the change in the dose is small with respect to the change in time, and thus the dose is easily controlled. Here, there is a progress in that the control precision of the dose is increased. However, in a plasma doping method that forms an impurity doped layer in the surface of the sample by generating plasma in the vacuum chamber and causing impurity ions in plasma to collide against the surface of the sample, the dose may be changed each time plasma is irradiated onto the silicon substrate, regardless of the same plasma condition, and reproducibility may be degraded. This is because, even though plasma is generated in the vacuum chamber in order to implant ions into the silicon substrate, the state in the vacuum chamber is changed for each time accordingly. Accordingly, it is difficult to adjust the dose with good reproducibility. In addition, since the state in the vacuum chamber is changed for each time, it is difficult to keep an in-plane dose of the silicon substrate uniformly. The dose can be made uniform by adjusting possible parameters or the shape of the apparatus, but the uniform dose cannot be repeatedly reproduced.
Patent Document 1: U.S. Pat. No. 4,912,065 (Specification)
Patent Document 2: JP-A-2004-179592
Patent Document 3: Japanese Patent No. 3340318
Non-Patent Document 1: Column of Shallow Junction Ion Doping of FIG. 30 of Front End Process in International Technology Roadmap for Semiconductors 2001 Edition (ITRS2001)
Non-Patent Document 2: Y. Sasaki, et al., Symp. on VLSI Tech. p 180 (2004)
Non-Patent Document 3: B. Mizuno et al., Plasma Doping into the side-wall of a sub-0.5 μm width Trench, Ext. Abs. of International Conference on SSDM, p. 317 (1987)
Non-Patent Document 4: B. Mizuno, et al., Plasma Doping for silicon, Surface Coating tech., 85, 51 (1996)
Non-Patent Document 5: B. Mizuno, et al., Plasma Doping of Boron for Fabricating the Surface Channel Sub-quarter micron PMOSFET, symp. VLSI Tech, p. 66 (1996)
As described above, it is known that the state in the vacuum chamber is changed for each time, but why the change occurs is not clear. As the result of various studies, the inventors have focused that a film is formed on an inner wall of a vacuum chamber as a plasma chamber and the state of the film is changed. Specifically, if a plasma doping treatment is repeated using mixture gas plasma of B2H6 gas and helium gas, the color concentration of the film and the formation area of the film are changed. That is, the inventors have focused that the thickness of the film becomes larger and the formation area of the film is increased. The invention has been finalized from this viewpoint. The inventors have supposed that the plasma concentration of the surface of the sample is changed when the film containing the impurity fixed to the inner wall of the vacuum chamber is attacked (sputtered) by ions in plasma or the variation is changed by the thickness of the film and the formation area of the film. In addition, the inventors have supposed that the change depends on the density of the impurity contained in a unit volume of the film.
According to the experiment result of the inventors, the dose of the impurity that is doped into the surface of the silicon substrate from a film containing the impurity fixed to the inner wall of the vacuum chamber is changed for each time.
The invention has been finalized in consideration of the above problems, and it is an object of the invention to provide a plasma doping method that can make a dose from a film to a silicon substrate for each time uniform even though a plasma processing is repeated.
The invention performs dose control and in-plane uniformity on the basis of a technical spirit that reverse the common knowledge of plasma doping up to now. According to the common knowledge of known plasma doping, the impurity is doped from ions, gas, radicals in plasma, and the material is supplied from a gas pipe connected to the vacuum chamber as gas. That is, the amount of the impurity contained in gas, the gas concentration or pressure, a mixture ratio of gas, and the like, determines the amount of the impurity doped into the surface of the semiconductor substrate. Accordingly, it is designed such that a plasma density or gas flow and a pressure distribution are made uniform on the surface of the semiconductor substrate. In addition, the adjustment of the dose is performed by adjusting the concentration of the impurity contained in gas to be supplied so as to adjust the concentration of the impurity contained in plasma or by adjusting an irradiation time of plasma.
In contrast, as the premise of the invention, it has been focused that a ratio of an impurity that is supplied from the gas pipe as gas, then plasmized, and subsequently doped into the surface of the semiconductor substrate is merely 15% to 30% of the total amount of the impurity to be doped by plasma doping. This is a numeric value that reverses the known common knowledge. In the related art, design of a process and the entire apparatus is performed on the basis of the spirit that the dose from gas plasma is a principal factor. In addition, in the invention, as the principal factor corresponding to remaining 85% to 70%, there is apparently the following phenomenon. That is, the film containing the impurity formed to be fixed to the inner wall of the vacuum chamber is exposed to plasma and sputtered while plasma doping is repeatedly performed. Then, the impurity doped into the film once is discharged in plasma again, and the discharged impurity is doped into the surface of the semiconductor substrate. The inventors have thought that a more accurate determination of the ratio of the factors relative to the dose requires future studies, and depends on the condition of plasma doping, but it is important to reduce the ratio of the dose from plasma that is considered as the principal factor in the related art. As considered in the related art, even though the parameters of plasma are adjusted, it can be seen that it is impossible to control the dose, stably keep repetitive uniformity, and perform a process with good reproducibility. That is, in order to control the dose, stably keep repetitive uniformity, and perform a process with good reproducibility, it is necessary to control a dose from the film containing the impurity as the principal factor and to secure stability. That is, it is necessary to adjust the film containing the impurity fixed to the inner wall of the vacuum chamber.
According to an aspect of the invention, there is provided a plasma doping method that places a sample on a sample electrode in a vacuum chamber, generates plasma in the vacuum chamber, and causes impurity ions in the plasma to collide against a surface of the sample so as to form an impurity doped layer in the surface of the sample. The plasma doping method includes a maintenance step of preparing the vacuum chamber having a film containing an impurity formed on an inner wall thereof such that, when the film containing the impurity fixed to the inner wall of the vacuum chamber is attacked by ions in the plasma, the amount of an impurity to be doped into the surface of the sample by sputtering is not changed even though the plasma containing the impurity ions is repeatedly generated in the vacuum chamber, a step of placing the sample on the sample electrode, and a step of irradiating the plasma containing the impurity ions so as to implant the impurity ions into the sample, and doping the impurity into the sample by sputtering from the film containing the impurity fixed to the inner wall of the vacuum chamber.
According to this configuration, the film is formed on the inner wall of the vacuum chamber such that the amount of the impurity to be doped into the sample by sputtering from the film containing the impurity of the inner wall of the vacuum chamber is not changed and then plasma doping is performed. Therefore, impurity doping can be stably performed with good reproducibility.
In the plasma doping method according to the aspect of the invention, the maintenance step may include, before forming the film containing the impurity, a substep of removing the film containing the impurity fixed to the inner wall of the vacuum chamber.
With this configuration, after the impurity stuck to the inner wall of the vacuum chamber is removed once, the film containing the impurity is formed again according to the conditions. Therefore, reliability can be improved.
In the plasma doping method according to the aspect of the invention, the sample may be a silicon substrate, and the film containing the impurity fixed to the inner wall of the vacuum chamber may be formed such that, when a dose of the impurity is the same level with a tolerance of ±10% even though the plasma containing the impurity ions is repeatedly generated in the vacuum chamber, the dose is made uniform in a surface of the silicon substrate.
With this configuration, the control can be performed with high accuracy.
The plasma doping method according to the aspect of the invention may further include a step of adjusting the shape of the inner wall of the vacuum chamber such that the amount of an impurity to be stuck to the inner wall of the vacuum chamber has a desired value.
For example, the shape of the inner wall of the vacuum chamber is adjusted such that, when the formation of the film is completed, the total distribution of the distribution of the impurity to be doped from the plasma containing the impurity ions and the distribution of the impurity to be doped by sputtering from the film containing the impurity fixed to the inner wall of the vacuum chamber is made uniform in the surface of the silicon substrate. It is important and more preferable to perform the adjustment such that the concentration is made uniform when the formation of the film is completed. In such a manner, uniform plasma doping can be realized with good repeatability. When the shape of the inner wall of the vacuum chamber is adjusted such that the distribution of the dose of the impurity is made uniform before the film is formed or when the film is being formed, since the state of the inner wall of the vacuum chamber, that is, the state of the film, is changed while plasma doping is repeated, it is difficult to reproduce uniformity.
The plasma doping method according to the aspect of the invention may further include a step of adjusting a gas supply method such that the amount of an impurity to be stuck to the inner wall of the vacuum chamber has a desired value.
For example, a film containing an impurity having a dark color, that is, a large thickness is likely to be formed in the vicinity of a gas jetting port. For this reason, the dose becomes large in a portion near the gas jetting port and becomes small in a portion distant from the gas jetting port. Accordingly, in-plane uniformity of the dose can be improved by adjusting the gas jetting port and the semiconductor substrate. For example, in-plane uniformity can be improved by moving, for example, rotating the semiconductor substrate with respect to the gas jetting port.
In the plasma doping method according to the aspect of the invention, the maintenance step may include a substep of providing the vacuum chamber, from which the film containing the impurity is removed, in a plasma doping apparatus and then generating the plasma containing the impurity ions in the vacuum chamber so as to form the film containing the impurity ions.
According to this configuration, a high-accurate impurity profile can be obtained with good controllability without using a special device.
In the plasma doping method according to the aspect of the invention, the step of forming the film containing the impurity ions may provide the vacuum chamber, from which the film containing the impurity is removed in the maintenance step, in a plasma doping apparatus separately provided in order to form the film and may generate the plasma containing the impurity ions in the vacuum chamber so as to form the film containing the impurity ions.
According to this configuration, desired control is performed using an additional device, and thus a high-accurate impurity profile can be obtained with good controllability.
The plasma doping method according to the aspect of the invention may further include a step of doping the impurity into the sample by sputtering from the film containing the impurity fixed to the inner wall of the vacuum chamber while measuring and managing a temperature of the inner wall of the vacuum chamber.
With this configuration, it has been found that the amount of the impurity to be doped from the film containing the impurity into the semiconductor substrate is changed by the temperature of the inner wall of the vacuum chamber. Accordingly, in order to keep the amount of the impurity constant, it is preferable to keep the temperature of the inner wall of the vacuum chamber constant. Further, in order to set the amount of the impurity to be doped from the film to a desired value, it is preferable to adjust the temperature of the inner wall of the vacuum chamber to a desired temperature.
Moreover, in the invention, a dummy chamber may be disposed in the vacuum chamber to cover the inner wall, and a film may be formed on the inner wall of the vacuum chamber. In vacuum equipment, there are many cases where the dummy chamber is called an inner chamber. In the invention, the formation of the film on the inner wall of the vacuum chamber, the studies of the shape of the inner wall, or the management of the temperature has been described. However, as for the inner chamber, the same effects can be obtained through the same studies. Therefore, the inner chamber still falls within the scope of the invention. In addition, the inner chamber does not have a function of holding the vacuum state, but can be simply detached, easily cleaned, and used as consumption goods. Accordingly, when the inner chamber is provided, it is desirable in that, instead of detaching and cleaning an expensive vacuum chamber, only the inner chamber can be detached and cleaned.
In the plasma doping method according to the aspect of the invention, the plasma may be plasma of gas containing boron.
According to this configuration, the boron film can be formed on the inner wall of the vacuum chamber. In addition, it is configured such that, when the film containing the impurity fixed to the inner wall of the vacuum chamber is attacked by the ions in the plasma, the amount of the impurity to be doped into the surface of the sample by sputtering is not changed even though the plasma containing the impurity ions are repeatedly generated in the vacuum chamber. Therefore, a high-accurate impurity profile can be obtained with good controllability, together with the impurity by sputtering.
In the plasma doping method according to the aspect of the invention, the gas containing boron may be gas of molecules having boron and hydrogen.
As the gas, BF3 or the like may be used, but, since F has a high sputter rate, it is difficult form a stable film. In order to form the stable film, gas having an atom having a low sputter rate is preferably used. In addition, if the sputter rate is high, the surface of the silicon substrate may be chipped off during the plasma doping treatment, a device cannot be manufactured according to design. Further, since the surface of the silicon substrate doped with the impurity is chipped off, impurity doping itself may not be performed with good controllability. Since hydrogen has a sputter rate less than F, if the gas of molecules having boron and hydrogen is used, a high-accurate impurity profile can be obtained with good controllability.
In the plasma doping method according to the aspect of the invention, the gas containing boron may be diborane (B2H6).
According to this configuration, B2H6 is industrially cheap, and is filled in a gas tank to be then transported and preserved in a gas state, which results in ease of handling. In addition, since only boron and hydrogen are contained, a sputter rate is low, and thus a high-accurate impurity profile can be obtained with good controllability.
In the plasma doping method according to the aspect of the invention, the plasma may be plasma of gas that is obtained by diluting gas of molecules having boron and hydrogen with rare gas.
According to this configuration, if the concentration of the gas containing boron is excessively high, the film may be easily separated. If the film is separated, particles may be generated to cause degradation of yield in manufacturing a semiconductor, which causes an inconvenience. Accordingly, if the gas concentration is lowered through the dilution with a different gas, a film that is rarely separated can be formed. As the dilution gas, rare gas having chemical stability is preferably used.
In the plasma doping method according to the aspect of the invention, the rare gas may be an atom having an atomic weight equal to or less than neon.
Among the rare gases, rare gas having a large atomic weight has a high sputter rate, and thus it is difficult to form a stable film. Further, it may chip off the surface of the silicon substrate. Therefore, the rare gas having an atomic weight smaller than neon is preferably used.
In the plasma doping method according to the aspect of the invention, the rare gas may be helium. In particular, helium has the smallest atomic weight and the lowest sputter rate among the rare gases. Therefore, a stable film is easily formed, and chipping of the silicon substrate can be suppressed to the minimum.
In the plasma doping method according to the aspect of the invention, the plasma may be plasma of gas that is obtained by diluting diborane (B2H6) with helium.
It is most preferable to use the gas diluted with helium such that the gas concentration of B2H6 becomes low.
In the plasma doping method according to the aspect of the invention, an implantation depth of boron may be in a range of 7.5 mm to 15.5 mm.
From the experiment result, if implantation energy corresponding to the implantation depth of boron ranging from 7.5 nm to 15.5 nm is used, it can be seen that a film containing boron is formed on the inner wall of the vacuum chamber such that sheet resistance is saturated. In addition, it can be seen that, when the formation of the film is completed, good in-plane uniformity is obtained.
In the plasma doping method according to the aspect of the invention, an implantation depth of boron may be equal to or less than 10 nm.
Under a low energy condition corresponding to the implantation depth of boron equal to or less than 10 nm, it is very difficult to obtain uniformity. However, from the experiment result, it can be seen that, according to the method of the invention, uniformity of 1.5% or less can be realized by adjusting the PD time.
In the plasma doping method according to the aspect of the invention, the plasma may use continuous plasma.
According to this configuration, uniformity of 1.5% or less can be realized by adjusting the PD time using the continuous plasma. In general, in plasma doping, there are developed a technology using continuous plasma and a technology using pulse plasma. When the pulse plasma is used, it has been reported that, in an implantation technology for a deep region more than approximately 20 nm, not the implantation to a shallow region as intended in the invention, uniformity and reproducibility are secured by plasma doping. However, as for the implantation into the shallow region, uniformity and reproducibility are insufficient. In contrast, in the invention, from various experiment results, in case of the implantation into the shallow region by the continuous plasma, uniformity and reproducibility can be secured.
According to another aspect of the invention, a plasma doping apparatus that performs the above-described plasma doping method includes a vacuum chamber, a sample electrode, a gas supply device that supplies gas into the vacuum chamber, an exhaust device that exhausts the vacuum chamber, a pressure control device that controls a pressure in the vacuum chamber, and a power supply for a sample electrode that supplies power to the sample electrode.
With this configuration, reproducibility of the dose of boron doped by plasma doping can be secured through the pressure control using the pressure control device.
The plasma doping apparatus according to another aspect of the invention may further include a plasma generation device that forms the film containing the impurity.
With this configuration, the state of the inner wall of the vacuum chamber can be easily controlled.
The plasma doping apparatus according to another aspect of the invention may further include a mechanism that adjusts a flow distribution of the gas to be supplied to the vacuum chamber such that the flow distribution of the gas can be adjusted after the film containing the impurity is formed, without exposing the inner wall of the vacuum chamber to atmosphere.
With this configuration, a desired internal state can be easily obtained in a short time without using an additional device and separately providing a preparation time forming the vacuum.
The plasma doping apparatus according to another aspect of the invention may further include a mechanism that adjusts a temperature of the inner wall of the vacuum chamber to a desired temperature.
The temperature control of the inner wall of the vacuum chamber can be realized by measuring the temperature using a temperature sensor and heating the inner wall using a heater. According to the experiment of the inventors, if the experiment is performed with no temperature control, the temperature of the inner wall of the vacuum chamber is initially at a room temperature, but it is increased to 40° C. to 90° C. when the plasma doping treatment is repeated. The increased temperature depends on the number of processing times or the conditions. Then, if the plasma doping treatment ends, the temperature is gradually decreased to the room temperature. That is, the temperature varies when the plasma doping treatment starts and when the plasma doping treatment is repeated. In addition, the temperature of the inner wall of the vacuum chamber is affected by a difference from an external temperature. Accordingly, it is preferable to adjust the temperature of the inner wall to a temperature that the inner wall naturally reaches when the plasma doping treatment is repeated, for example, a desired temperature of 40° C. to 90° C. Therefore, the amount of the impurity to be doped from the film can be adjusted to a desired value. Further, it is more preferable to adjust the temperature of the inner wall to a desired temperature 50° C. to 70° C. As a result, since the adjustment to a temperature that the inner wall naturally reaches under more plasma doping conditions can be performed, good repeatability can be obtained.
According to still another aspect of the invention, there is provided a silicon substrate that has a diameter 300 mm and into a surface of which boron is doped by plasma doping using continuous plasma containing boron. In this case, a profile of doped boron has a depth ranging 7 nm to 15.5 nm with a boron concentration of 5×1018 cm−3, steepness of the depth profile of boron is in a range of 1.5 nm/dec 3 nm/dec upon evaluation at a distance where the boron concentration is lowered from 1×1019 cm−3 to 1×1018 cm−3, and a dose of boron has a standard deviation of 2% or less at a surface excluding an end 3 mm of the silicon substrate. Among many products that can be manufactured using the method according to the invention, when the above-described substrate is manufactured, the following marked effects can be obtained. If boron is doped at the depth of the above range, a very fine source-to-drain extension electrode of a MOSFET of a 65 nm node to 22 nm node can be formed. Further, when boron is doped at the steepness of the above range, a very fin drain current of the MOSFET can be increased. When boron is doped by plasma doping, the very fine electrode of the MOSFET can be produced with good productivity. In addition, since in-plane uniformity of the dose can be improved by the 300 mm substrate, productivity and yield can be improved. Moreover, this is verified by way of the example using a silicon substrate as the semiconductor substrate. A germanium substrate or a strained silicon substrate may be used since an atom used in the germanium substrate or the strained silicon substrate has an atomic weight not different only negligibly from the silicon atom and thus it can be supposed that the same effects are obtained.